1. Technical Field
The following description relates to an organic light emitting diode display device which compensates a threshold voltage of a driving TFT.
2. Discussion of the Related Art
There has been an increasing demand for a display device for displaying an image together with the growth of an information-oriented society, and various flat panel displays such as liquid crystal displays (LCDs), plasma display panels (PDPs), and organic light emitting diodes (OLEDs) have been widely used in recent years. Among the flat panel displays, organic light emitting diode display devices are driven at a low voltage, are thin, have a wide viewing angle and a quick response speed. Among the organic light emitting diode display devices, active matrix type organic light emitting diode display devices have been widely used since they have a plurality of pixels disposed in a matrix form to display an image.
A display panel of the active matrix type organic light emitting diode display device comprises a plurality of pixels arranged in matrix. Each pixel includes a scan TFT (thin film transistor) supplying data voltage from data line in response to scan signal from the scan line and a driving TFT controlling the amount of current supplied to an organic light emitting diode in response to the data voltage supplied to a gate electrode. At this point, the current Ids between a drain electrode and a source electrode of the driving TFT supplied to the organic light emitting diode can be represented by Equation 1:Ids=k′·(Vgs−Vth)2  [Equation 1]
In equation 1, k′ indicates a proportionality factor determined by the structure and physical properties of the driving TFT, Vgs indicates a voltage difference between the gate electrode and source electrode of the driving TFT, and Vth indicates a threshold voltage of the driving TFT.
Meanwhile, due to a threshold voltage shift caused by deterioration of the driving TFT, the threshold voltage Vth of the driving TFT of each of the pixels has a different value. The current Ids between the drain electrode and source electrode of the driving TFT is dependent on the threshold voltage Vth of the driving TFT. Thus, the current Ids between the drain electrode and source electrode of the driving TFT of each pixel is varied even if the same data voltage is supplied to each of the pixels. Accordingly, there arises the problem that the luminance of light emitted from the organic light emitting diode of each of the pixels is varied even if the same data voltage is supplied to each of the pixels. To solve this problem, various types of pixel structures for compensating the threshold voltage of the driving TFT of each of the pixels have been proposed.
However, recently, the organic light emitting diode display device is driven by a high frequency in order to implement a 3D image or improve an image quality. Accuracy of sensing a threshold voltage gets lower because a period for sensing a threshold voltage is shortened due to the high frequency driving.
Also, recently, the organic light emitting diode display device has a big size and a high resolution. Due to a big size and a high resolution, line resistance increases because a length of the line becomes longer. Finally, RC delay can be occurred. The period for sensing a threshold voltage is shortened due to RC delay. Therefore, accuracy of sensing a threshold voltage gets lower.